The present invention relates to a process for the purification of a heteropolyacid, and in particular, to a process for the purification of a silicotungstic acid.
It is well known that heteropolyacids substantially free of extraneous cations (hereafter xe2x80x9cfree heteropolyacidsxe2x80x9d) are valuable chemical compounds which can be used as acid catalysts in organic reactions such as the addition reaction of lower aliphatic carboxylic acids to olefins to form the corresponding esters. One such method is described in our EP-A-0757027 which describes the use of free heteropolyacids as catalysts for the production of aliphatic esters such as ethyl acetate in an addition reaction as described above. Methods of synthesising heteropolyacids are also well known. For instance, such processes are described e.g. by North, E O in xe2x80x9cOrganic Synthesisxe2x80x9d, 1, page 129 (1978), Editor H S Booth, Robert E Krelger publishing company Huntington, N.Y.) and by Tatsuhiko, H et al in Kogyo Kagaku Zasshi, 72 (9), 1945-48 (1969).
Known methods for the preparation of heteropolyacids generally involve three stages, namely:
a. A reaction stage in which the tungstosilicic acid is formed by the addition of hydrochloric acid to a refluxing aqueous solution of sodium tungstate and sodium silicate. Upon neutralisation and hydrolysis, the reaction results in the formation of a brine solution containing a tungstosilicic acid Keggin structural unit. Overall, the reaction can be summarized as follows:
12Na2WO4+Na2SiO3+HClxe2x86x92H4SiW12O40.nH2O+NaCl+H2O 
The product is filtered, acidified and cooled prior to purification.
b. A product recovery and purification stage in which the cooled product solution from (a) is purified by at least one liquid/liquid extraction step using an organic solvent. The solvent is such that it is capable of forming a solvent/heteropolyacid complex which is largely insoluble in the acidified aqueous brine. The resulting solvent/complex phase enables the disengagement of the contaminating inorganic salts from the brine solution.
c. A product regeneration stage in which an aqueous solution of the heteropolyacid is regenerated from the solvent/complex phase produced in step b). This regeneration is achieved by adding water to displace the solvent involved in the formation organic/complex phrase. The organic solvent is removed from the mixture by distillation. The organic solvent is conveniently removed as a solvent/water azeotrope. This azeotrope may form a two phase mixture on cooling. The denser of these two phases is an aqueous phase, which may be recycled to the reboiler. The less dense phase is an organic phase, which may be recovered, for example, for reuse. The amount of water added and removed can be adjusted to yield a 20-80%w/w aqueous solution of heteropoly acid.
The resultant concentrated aqueous solution of the heteropolyacid product can then be used to impregnate an inorganic support.
The known synthetic methods referred to above mention the use of diethyl ether, methyl ether ketone and ethyl acetate as possible solvents for purification of the product. We have found that all these materials have inherent disadvantages. For instance, diethyl ether has a high volatility and low auto-ignition point;
methyl ethyl ketone gives rise to colour formation due to desired acid catalysing an aldol type reaction with the solvent ketone; and esters such as ethyl acetate have a propensity to undergo hydrolysis in the presence of the heteropolyacid being synthesised and any neutralising acids present therein.
It has now been found that these disadvantages can be mitigated if a suitable solvent is used during the purification stages.
According to a first aspect of the present invention, there is provided a process for purifying a heteropolyacid which comprises:
subjecting an aqueous solution comprising (i) the heteropolyacid and (ii) salt impurities to at least one liquid/liquid extraction step with an organic solvent,
characterised in that the organic solvent comprises a dihydrocarbyl ether having at least 5 carbon atoms.
In other words, the dihydrocarbyl ether is such that the total number carbon atoms in the two hydrocarbyl units, one on either side of the ethereal oxygen atom, is at least 5.
The liquid/liquid extraction step enables the heteropolyacid to form a complex in situ with the organic solvent. This allows the extract to separate into two or more phases including inter alia a dense organic phase, which generally settles at or towards the bottom of the separation vessel. This phase comprises the dissolved complex of the heteropolyacid.
Preferably, the dense organic phase is recovered. This may be mixed with water to form a separate layer of a dilute aqueous mixture, from which a concentrated 20-80% w/w solution of the heteropolyacid substantially free of organic impurities may be isolated.
The starting aqueous solution of heteropolyacid may be prepared by reacting (i) an aqueous solution of one or more alkali or alkaline earth metal salts selected from a tungstate and a molybdate with (ii) an alkali or an alkaline earth metal silicate or an alkali or an alkaline earth metal phosphate. Preferably, this reaction is carried out under reflux. The ensuing reaction produces an impure aqueous solution comprising the heteropolyacid of the chosen salt and any other salt formed in situ or used as a reactant. This solution may then be filtered, cooled and subjected to the liquid/liquid extraction step of the first aspect of the present invention.
Preferably, the heteropolyacid in the aqueous solution is a silicotungstic acid.
According to a second aspect of the present invention, there is provided a process for the synthesis of a heteropolyacid substantially free of extraneous cations, said process comprising:
a. reacting (i) an aqueous solution of one or more alkali or alkaline earth metal salts selected from a tungstate and a molybdate under reflux with (ii) an alkali or an alkaline earth metal silicate or an alkali or an alkaline earth metal phosphate in the presence of an acid to form in an impure aqueous solution comprising the heteropolyacid of the chosen salt and any other salt formed in situ or used as a reactant;
b. filtering and cooling the impure aqueous solution comprising the heteropolyacid;
c. purifying the cooled, impure aqueous solution from (b) by at least one liquid/liquid extraction with an organic solvent to enable the heteropolyacid to form a complex in situ with the organic solvent and allowing the extract to separate into two or more phases including inter alia a dense organic phase which comprises the dissolved complex of the heteropolyacid and which dense organic phase is below the aqueous phase;
d. recovering the dense organic phase from (c) and mixing it thoroughly with water to form a separate layer of a dilute aqueous mixture and
e. recovering a concentrated 20-80%w/w aqueous solution of the heteropolyacid substantially free of organic impurities from the dilute aqueous mixture,
wherein the organic solvent used in the liquid/liquid extraction step (c) comprises a dihydrocarbyl ether having at least 5 carbon atoms.
Preferably, the organic solvent used in the liquid/liquid extraction step (c) is a dihydrocarbyl ether having at least 5 carbon atoms.
The dihydrocarbyl ether may have 5-20 carbon atoms, and preferably, has 5-10 carbon atoms. The dihydrocarbyl ether may be such that at least one of the hydrocarbyl groups is a branched chain alkyl group. Preferably, the branched chain alkyl group is t-butyl or amyl. At least one of the hydrocarbyl groups in the ether may be a methyl, ethyl or propyl group. Preferably, this hydrocarbyl group is a methyl or ethyl group, and most preferably, it is a methyl group. It may be advantageous to employ ethers which have different hydrocarbyl groups on either side of the ethereal oxygen. For example, the ether may have a methyl group on one side of the ethereal oxygen, and a butyl or amyl group on the other.
The ether used should be substantially immiscible with water. In other words, the ether should be substantially insoluble in water, and water should be substantially insoluble in the ether. The ether when complexed with HPA should also be immiscible with water in order to facilitate the removal of any salt from the extract.
Suitable ethers generally have a boiling point above 40xc2x0 C., preferably between 40xc2x0 C. and 60xc2x0 C., more preferably between 50xc2x0 C. and 60xc2x0 C., for example, between 55 and 56xc2x0 C. In some applications, ethers having boiling points of above 90xc2x0 C. may also be suitable.
Suitable ethers include propyl butyl ether (e.g. propyl t-butyl ether), ethyl propyl ether, dipropyl ether (e.g. di-n-propyl ether (bp 88-90xc2x0 C.) and di-isopropyl ether), butyl glycol ether, di-butyl ether (e.g. disobutyl ether, di t-butyl ether, di n-butyl ether), methyl butyl ether (e.g. methyl n-butyl ether [bp 70-71xc2x0 C.], methyl sec butyl ether [bp 75xc2x0 C.], methyl tertiary butyl ether [bp 55-56xc2x0 C.]), methyl amyl ether (e.g. 4-tert-amyl methyl ether [bp 85-86xc2x0 C.]), butyl ethyl ether (e.g. n-butyl ethyl ether [bp 91-92xc2x0 C.], tert-butyl ethyl ether (bp 72-73xc2x0 C.)), and propyl butyl ether. Of these, methyl tertiary amyl ether and more particularly, methyl tertiary butyl ether (hereafter xe2x80x9cMTBExe2x80x9d) are preferred. An advantage of using such ethers as solvents is that they would reduce the total organic content of any effluent due to their low water solubility.
Step a) may be preceded by the preparation of an aqueous solution of at least one alkali or alkaline earth metal salt of a tungstate or a molybdate. Such solutions may be prepared using demineralised water to minimise the contamination of the final heteropolyacid desired by extraneous ions. Preferably, a solution of sodium molybdate or sodium tungstate is prepared.
The aqueous solution of at least one alkali or alkaline earth metal salt of a tungstate or a molybdate is then refluxed with an alkali or an alkaline earth metal silicate or phosphate (step a). The silicate or phosphate may be used as a solid or as an aqueous solution. Preferably, a silicate is used.
The ensuing reaction produces a mixture comprising an impure solution of the mixed heteropolyacid of the chosen salt, which may be generated initially as the salt of a partially acidified heteropolyacid (e.g. H2Na2-heteropolyacid) or the fully acidified heteropolyacid (H4-heteropolyacid). This resultant solution is then filtered, cooled, and preferably, acidified.
The reaction of step a) is carried out in the presence of an acid, which neutralises the reaction mixture. The heteropolyacid unit is believed to assemble during this neutralisation step. Suitable acids for the neutralisation step include mineral acids such as hydrochloric acid.
Preferably, acid is introduced into the reaction mixture of step a) through the reflux flow. The addition of acid is controlled so as to prevent the precipitation of the compounds such as e.g. tungstic acid.
More preferably, the addition of hydrochloric acid to the reaction is carried out in two stages. Firstly, as mentioned above, during step a) so as to neutralise the reaction mixture, and secondly, before the refluxed reaction mixture is liquid/liquid extracted in step c). This second acidification may be carried out as the mixture of step a) is cooled and filtered, but preferably, is carried out after the cooling and filtration step. The purpose of the second acid addition stage is (i) to ensure that the reactant salts are appropriately hydrolysed, (ii) to convert the heteropolyacid present in the reaction mixture to the H4 form, and (iii) to prevent ionisation in the aqueous solution. Since ionisation of the heteropolyacid in solution aids the dissolution thereof in water, the acidification of the solution by addition of an acid, and in particular, a strong mineral acid, suppresses this tendency of the heteropolyacid to ionise. This not only decreases its water solubility but also allows it to be extracted more easily by solvent extraction.
The acidified, cooled and filtered solution is then subjected to at least one liquid/liquid extraction step (step c). Preferably, a plurality of such extraction steps are employed. The liquid/liquid extraction is carried out using an organic solvent, which complexes with the heteropolyacid in situ. It should be noted that not all solvents are capable of forming such a complex; for example, cyclohexane is incapable of forming such a complex with the heteropolyacid.
The liquid/liquid extract is then allowed to separate into two or more phases. The complex of the heteropolyacid is soluble in the extracting organic solvent, and forms a solution which separates as a dense organic phase.
In certain embodiments of the invention, the extract separates into three phases: an upper organic phase which comprises mainly the solvent with traces of heteropolyacid, a middle aqueous phase which comprises mainly water and some of the acids and salts used or formed during the synthesis reaction and finally, a lower dense organic phase which contains the desired heteropolyacid complex of interest. The liquid/liquid extraction behaviour of such systems is unusual in that a complex of the heteropolyacid and the extracting solvent is formed in the lower dense organic phase when the components are mixed together.
The efficiency of the extraction will depend upon the extent of disengagement of the phases in the extract. In particular, the organic phase comprising the complex of the organic solvent and the heteropolyacid may appear to be initially hazy due to entrapped salts/aqueous phase of the reactant inorganic compounds and any salts formed in situ during the reaction. Such a haze may be removed using, for example, demisting treatments. Suitable demisting treatments include centrifugation and filtration methods.
The solution comprising the complex of the heteropolyacid is separated from the other phase(s). Preferably, this solution is admixed with water, which may advantageously be demineralised and/or de-ionised prior to the admixing step. The organic solvent may suitably be removed from the resultant admixture by azeotropic distillation in a column. The preferred organic solvents can form decanting mixtures, which allow facile separation of a bottom aqueous layer from the top organic phase. The aqueous layer can be returned to the distillation column or removed as a column heads take off. The amount of water employed to prepare the admixture can be adjusted prior to the distillation; similarly, the amount of water remaining after the distillation can also be adjusted to yield a solution of heteropolyacid 20-80%w/w of said heteropolyacid.
An alternative method of preparing an aqueous solution of the free heteropolyacid is to distil the organic phase under vacuum to remove the solvent and then take up the residue in water. Care, however, should be taken with vacuum drying as extended heating under vacuum even mild conditions (e.g. 100xc2x0 C., 16 hrs, 0.01 mmHg) can lead o some heteropolyacid loss by generation of by-products.
The term xe2x80x9cheteropolyacidxe2x80x9d as used herein and throughout the specification is meant to include the free acids. The heteropolyacids may be used to prepare esterification catalysts and olefin addition catalysts. When employed for such applications, the heteropolyacids may be used as free acids and or partial salts. The synthesis of heteropolyacids in accordance with the present invention. Allows the purity of such salts to be controlled.
Typically, the heteropolyacid, or the anionic component of its corresponding salt comprises 2-18 oxygen-linked polyvalent metal atoms, which are called peripheral atoms. These peripheral atoms surround one or more central atoms in a symmetrical manner. The peripheral atoms are usually one or more of molybdenum, tungsten, vanadium, niobium, tantalum and other metals. The central atoms are usually silicon or phosphorus but can comprise any one of a large variety of atoms from Groups I-VIII in the Periodic Table of elements. These include, for instance, cupric ions; divalent beryllium, zinc, cobalt or nickel ions; trivalent boron, aluminium, gallium, iron, cerium, arsenic, antimony, phosphorus, bismuth, chromium or rhodium ions; tetravalent silicon, germanium, tin, titanium, zirconium, vanadium, sulphur, tellurium, manganese nickel, platinum, thorium, hafnium, cerium ions and other rare earth ions; pentavalent phosphorus, arsenic, vanadium, antimony ions; hexavalent tellurium ions; and heptavalent iodine ions. Such heteropolyacids are also known as xe2x80x9cpolyoxoanionsxe2x80x9d, xe2x80x9cpolyoxometallatesxe2x80x9d or xe2x80x9cmetal oxide clustersxe2x80x9d. The structures of some of the well known anions are named after the original researchers in this field and are known e.g. as Keggin, Wells-Dawson and Anderson-Evans-Perloff structures.
Heteropolyacids usually have a high molecular weight e.g. in the range from 700-8500 and include dimeric complexes. They have a relatively high solubility in polar solvents such as water or other oxygenated solvents, especially if they are free acids and in the case of several salts, and their solubility can be controlled by choosing the appropriate counter-ions. Specific examples of heteropolyacids that may be synthesised in the present invention include:
The present invention is particularly suitable for the synthesis and purification of silicotungstic acid.
The heteropolyacids purified or synthesised by the present invention may be impregnated into a support. This may be achieved by dissolving the heteropolyacid in distilled water, and then adding the support to the aqueous solution so formed. The support is suitably left to soak in the acid solution for a duration of several hours, with periodic manual stirring, after which time it is suitably filtered using a Buchner funnel in order to remove any excess acid.
The wet catalyst thus formed is then suitably placed in an oven at elevated temperature for several hours to dry, after which time it is allowed to cool to ambient temperature in a desiccator. The catalyst can also be dried suitably by using a flow of heated gas such as e.g. nitrogen or air. The catalyst loading in g/liter was determined by deducting the weight of the support used from the weight of the catalyst on drying.
Alternatively, the support may be impregnated with the catalyst using the incipient wetness technique and dried by using a flow of heated gas such as e.g. nitrogen or air.
This supported catalyst (measured by weight) can then be used in processes such as e.g. the hydration of olefins to form alcohols or the addition of aliphatic monocarboxylic acids to olefins to form the corresponding ester. The amount of heteropolyacid deposited/impregnated on the support for use in the addition reaction for making esters is suitably in the range from 10 to 60% by weight, preferably from 20 to 50% by weight, more preferably from 20-35% by weight (corresponding to a loading in the range of about 100-215 g/liter) based on the total weight of the heteropolyacid and the support.
The present invention is further illustrated with reference to the following Examples: